Beholder

The beholder gains profiency in one saving throw of your choice: Constitution, Intelligence, or Charisma.

In addition, the cohort gains proficiency in two skills of your choice from the following list: Arcana, History, Insight, Intimidation, Investigation, or Persuasion.

(The above list assumes the cohort has the Beholder race--if not, then add Perception to the list of options.)

Starting at level 2, the beholder learns to use its most feared ability: eye rays. The save DC against your eye rays (from both the Lesser Eye Ray and Greater Eye Ray abilities) is calcuated as follows:

As an action during your turn, you may roll 1d4 and fire the associated eye ray listed below, choosing one target you can see within 60 feet of you:

At 3rd level, your proficiency bonus is doubled for the purposes of your passive perception score, as well as for any ability check you make that uses the Perception skill.

When you reach 4th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.

At 5th level, your hide becomes more resilient. Each beholder is unique, so this armor increase may be due to a thicker hide, chitinous plates along your body, or harder skin (as a few possible examples). When unarmored, your armor class is now 14.

Starting at 6th level, whenever you fire a Lesser or Greater Eye Ray (see below) as an action, you may fire another Lesser Eye Ray as a bonus action. You may choose to target the same or a different creature within range that you can see.

At 7th level, you finally grow enough to become a larger size category entirely. You are now Medium size. The range of your Lesser and Greater Eye Ray abilities is now 90 feet. Your darkvision (from the beholder race) now extends to 90 feet.

In addition, your bite attack now deals 2d6 piercing damage plus your strength modifier. From now on, your natural weapons now count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

When you reach 8th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.

At 9th level, your natural armor becomes 16.

Starting at 9th level, your innate magical abilities allow you to change your physical form. You are able to cast the alter self spell without any material components and without using a spell slot. Whenever you use this ability, you can take on the form of any Small or Medium-sized humanoid. You cannot use your Lesser Eye Ray or Greater Eye Ray abilities while in humanoid form, though you may still use your Antimagic Cone ability (see below). Note that, when you use the Antimagic Cone ability in humanoid form, you must close all of this form's eyes to suppress the ability

Due to the otherworldly influence you exert on your surroundings, however, there is always something a little unsettling about you regardless of your outward appearance. When in humanoid form, a creature observing you closely (DM's discretion) may make a Perception check against a DC of 18. Whenever a creature succeeds on this check, they recognize that your appearance is close, but not quite right for a member of your species. The creature may not know how they are aware of this fact--but, without a doubt, they believe there is something inherently disturbing about you.

At 10th level, you learn how to use the power of the Far Realm to alter the world in hazy, dreamlike ways. However, in this regard you are somewhat unique. Many of your kind are able to manifest powers of the Far Realm, but only when in the lairs they inhabit. Though you cannot use these abilities as often as others of your kind, you have learned to exert your will on reality no matter where you are.

During your turn, you can use a bonus action to cause one of the following effects:

You may use each of these abilities once. You recover all expended use of these abilities upon completion of a short or long rest.

As you mature, more of your eye stalks develop offensive abilities. Starting at 11th level, you can hinder enemies with a wider array of effects... much to their displeasure.

As an action during your turn, you may roll 1d10 and fire the associated eye ray listed below, choosing one target you can see within 60 feet of you:

When you reach 12th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.

At 13th level, your vision of the world begins to alter reality in subtle ways-- specifically, your large, central eye can now cancel out magical effects that it can see.

As a bonus action during your turn, your central eye creates an area of antimagic, as in the antimagic field spell, in a 90 foot cone. At the start of each of your turns, you decide which way the cone faces and whether the cone is active (closing your central eye supresses the cone until you open your eye again or until the duration expires). The area works against your own eye rays.

This ability lasts for 10 minutes. You may use this ability once, and regain any expended use upon completion of a short or long rest.

Starting at 14th level, whenever you use your Lesser Eye Ray or Greater Eye Ray abilities, you can choose to reroll the result. You must accept the result of the new roll.

Once you use this ability, you cannot do so again until the beginning of your next turn.

At 15th level, your mass has grown to again meet a new threshold. You have come a long way since your earliest days, and could no longer be confused for your lesser kin. You are now a Large-sized creature, taking up a 10 by 10 foot space. Your darkvision, as well as the range of your Lesser and Greater Eye Ray abilities now extend to 120 feet. Your Antimagic Cone ability is now a 150 foot cone.

In addition, you now add your proficiency bonus to the damage you deal with your bite attack.

When you reach 16th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.

At 17th level, your natural armor becomes 18.

At 18th level, your array of eyes ensure that you are aware of threats before they can strike. You may act normally during a surprise round as long as you are not incapacitated.

In addition, creatures cannot gain advantage on attack rolls against you unless their number of maximum hit dice exceeds your own, or unless you are suffering from the blinded, paralyzed, restrained, stunned or unconscious conditions.

When you reach 19th level, you can increase one ability score of your choice by 2, or you can increase two ability scores of your choice by 1. As normal, you can't increase an ability score above 20 using this feature.

At 20th level, your strengths as a beholder finally reach their full potential. Your allies (assuming you have any) should consider themselves fortunate they have the greatest of all beholderkin fighting alongside them. Your enemies, however, may not be so lucky.

Your hit dice all become d10s. This causes your current and maximum hit points to increase by 20 immediately. Further, your hit dice all become d10s for the purpose of restoring hit points during a short rest.

In addition, whenever you use your Greater Eye Ray ability, you can choose to fire one or two Greater Eye Rays, choosing one to two targets you can see within 120 feet of you.